Complex pharmacology of opioid drugs cannot be fully understood without deciphering molecular and structural details of drug-receptor interactions and functional mechanisms. This knowledge is critical for development of new generation of effective and side-effect free therapies for pain control, mood disorders and other indications. A new unique opportunity for such structure-based analysis is provided by recently solved crystal structures of all four subtypes of opioid receptors and continuing efforts for crystallographic characterization of their complexes with different ligands. These structures create a reliable 3-dimensional framework for computational analysis and modeling of ligand-receptor interactions and their functional and pharmacological consequences. The overall goal of this project is to decipher structural basis of opioid drugs action at atomic level and apply this knowledge to rationally design new tool compounds and candidate leads by using state ofthe art computational technologies for molecular modeling, ligand docking and virtual ligand screening. The specific Aims are (i) Development of 3D structural models of opioid receptors complexes with all known drugs and drug candidates, as well as systematic mapping of the ligand-receptor interactions for determinants of selectivity to a specific opioid subtype or specific function (agonism, antagonism, biased signaling); (ii) Virtual ligand screening of drug-like compounds against all opioid receptor subtypes in different functional states for discovery of new opioid ligand chemotypes; (iii) Computer-assisted structure- based design of lead compounds, including allosteric and bitopic ligands, for development new tool compounds for opioid receptors with desired functional and pharmacologic profile. Each Aim involves thorough experimental validation of the proposed hypotheses, as well as synthesis and biochemical/biophysical testing of the candidate ligands for binding and signaling properties by the collaborative Projects and the Chemistry Core. Receptor complexes for new compounds with most interesting structural and functional features will be tried for crystallization. Success of the program will not only result in new insights into drug action and lead to new opioid ligands with desired properties as tool compounds, but it will establish a platform for rational structure-based discovery of safer opioid therapies. RELEVANCE (See instructions): Development of new generation opioid medications with reduced risks of addiction, tolerance and other unwanted side effects is of paramount importance for pain and anxiety relief therapies. This project will combine molecular modeling with in vitro and in vivo experimental testing to decipher basic mechanisms of opioid drug action on their receptors and suggest molecular ways to separate their pain and anxiety relief effects from side effects. New knowledge will be applied to design of chemical compounds and peptides with specific functional features for pain research and pave the path for new candidate therapies.